The invention relates generally to screws and coupling element assemblies for use with orthopedic fixation systems. More particularly, the invention relates to a screw and coupling element assembly, for use with an orthopedic rod implantation apparatus, that includes a locking element that simultaneously locks a head of the screw within a seat of the coupling element so that a shaft of the screw is fixed at an angle relative to the coupling element and a rod, of the orthopedic rod implantation apparatus, disposed within a rod-receiving channel of the locking element is fixed relative to the coupling element.
The bones and connective tissue of an adult human spinal column consists of more than twenty discrete bones coupled sequentially to one another by a tri-joint complex which consist of an anterior disc and the two posterior facet joints, the anterior discs of adjacent bones being cushioned by cartilage spacers referred to as intervertebral discs. These more than twenty bones are anatomically categorized as being members of one of four classifications: cervical, thoracic, lumbar, or sacral. The cervical portion of the spine, which comprises the top of the spine, up to the base of the skull, includes the first seven vertebrae. The intermediate twelve bones are the thoracic vertebrae, and connect to the lower spine comprising the five lumbar vertebrae. The base of the spine is the sacral bones (including the coccyx). The component bones of the cervical spine are generally smaller than those of the thoracic and lumbar spine.
Referring now to FIGS. 1, 2, and 3, top, side, and posterior views of a vertebral body, a pair of adjacent vertebral bodies, and a sequence of vertebral bodies are shown, respectively. The spinal cord is housed in the central canal 10, protected from the posterior side by a shell of bone called the lamina 12. The lamina 12 includes a rearwardly and downwardly extending portion called the spinous process 16, and laterally extending structures which are referred to as the transverse processes 14. The anterior portion of the spine comprises a set of generally cylindrically shaped bones which are stacked one on top of the other. These portions of the vertebrae are referred to as the vertebral bodies 20, and are each separated from the other by the intervertebral discs 22. The pedicles 24 comprise bone bridges which couple the anterior vertebral body 20 to the corresponding lamina 12.
The spinal column of bones is highly complex in that it includes over twenty bones coupled to one another, housing and protecting critical elements of the nervous system having innumerable peripheral nerves and circulatory bodies in close proximity. In spite of these complexities, the spine is a highly flexible structure, capable of a high degree of curvature and twist in nearly every direction. Genetic or developmental irregularities, trauma, chronic stress, tumors, and disease, however, can result in spinal pathologies which either limit this range of motion, or which threaten the critical elements of the nervous system housed within the spinal column. A variety of systems have been disclosed in the art which achieve this immobilization by implanting artificial assemblies in or on the spinal column. These assemblies may be classified as anterior, posterior, or lateral implants. As the classifications suggest, lateral and anterior assemblies are coupled to the anterior portion of the spine, which is the sequence of vertebral bodies. Posterior implants generally comprise pairs of rods, which are aligned along the axis which the bones are to be disposed, and which are then attached to the spinal column by either hooks which couple to the lamina or attach to the transverse processes, or by screws which are inserted through the pedicles.
xe2x80x9cRod assembliesxe2x80x9d generally comprise a plurality of such screws which are implanted through the posterior lateral surfaces of the laminae, through the pedicles, and into their respective vertebral bodies. The screws are provided with upper portions which comprise coupling elements, for receiving and securing an elongate rod therethrough. The rod extends along the axis of the spine, coupling to the plurality of screws via their coupling elements. The rigidity of the rod may be utilized to align the spine in conformance with a more desired shape.
It has been identified, however, that a considerable difficulty is associated with inserting screws along a misaligned curvature and simultaneously exactly positioning the coupling elements such that the rod receiving portions thereof are aligned so that the rod can be passed therethrough without distorting the screws. Attempts at achieving proper alignment with fixed screws is understood to require increased operating time, which is known to enhance many complications associated with surgery. Often surgical efforts with such fixed axes devices cannot be achieved, thereby rendering such instrumentation attempts entirely unsuccessful.
The art contains a variety of attempts at providing instrumentation which permit a limited freedom with respect to angulation of the screw and the coupling element. These teachings, however, are generally complex, inadequately reliable, and lack long-term durability. These considerable drawbacks associated with prior art systems also include difficulty in properly positioning the rod and coupling elements, and the tedious manipulation of the many parts that are used in the prior art to lock the rod, the screw, and the coupling element in position once they are properly positioned. It is not unusual for displacement to occur as these parts are manipulated to lock the elements, which is clinically unacceptable, and repeated attempts at locking the elements in proper position must be made to remedy this displacement.
There is, therefore, a need for a screw and coupling element assembly which provides a polyaxial freedom of implantation angulation with respect to rod reception. There is also a need for such an assembly which comprises a reduced number of elements, and which correspondingly provides for expeditious implantation. There is also a need for such an assembly that provides reduced difficulty in locking steps to prevent unwanted displacement of the elements prior to locking. There is also a need for an assembly which is reliable, durable, and provides long term fixation support.
The invention provides a screw and coupling element assembly for use with an orthopedic rod implantation apparatus. The assembly includes a securing element such as, for example, a screw, that has a head and a shaft that extends from the head. Preferably, the head has an engagement surface that can be engaged by a screwdriving tool. Preferably, the head also has a curvate proximal portion from which the shaft extends.
The screw and coupling element assembly further includes a coupling element and a locking element. The coupling element has a seat within which the head of the screw can be seated such that the shaft of the screw protrudes from the coupling element. The locking element can be mated with the coupling element and thereafter can be selectively moved through a plurality of positions including an unlocked position and a locked position. When the locking element is in the unlocked position, the locking element presents a rod-receiving channel and the head of the screw is movable in the seat of the coupling element, such that the shaft of the screw can be directed in a plurality of angles relative to the coupling element. When the locking element is in the locked position, a rod disposed within the rod-receiving channel is fixed relative to the coupling element, and the head of the screw is immovable in the seat of the coupling element, such that the shaft of the screw is fixed at an angle relative to the coupling element. More particularly, when the locking element is in the locked position, the locking element is compression locked within the coupling element, the head of the screw is compression locked within the seat of the coupling element, and the rod is compression locked within the rod-receiving channel.
Preferably, at least one feature on the locking element can be used to move the locking element through the positions when the locking element is disposed within the coupling element. Preferably, the feature comprises an engagement surface that can be engaged by a corresponding surface of a tool, such that the tool can engage the engagement surface of the feature and effectively move the locking element. Preferably, the corresponding surface of the tool can engage the engagement surface of the feature despite the presence of the rod in the rod-receiving channel.
Preferably, the seat of the coupling element can be defined by a bore in the coupling element and a socket defined by a curvate volume adjacent the bore. The curvate volume corresponds to the curvate proximal portion of the head of the screw such that the curvate proximal portion can be nested in the socket. When the curvate proximal portion of the head of the screw is nested in the socket, the shaft of the screw protrudes from the bore and the curvate proximal portion cannot pass fully through the bore. When the locking element is in the unlocked position, the curvate proximal portion of the head of the screw can rotate and angulate in the socket such that the shaft of the screw can be directed through a range of angles relative to the coupling element. When the locking element is in the locked position, the curvate proximal portion of the head of the screw is immovable in the socket such that the shaft of the screw is fixed at an angle relative to the coupling element.
Preferably, the locking and unlocking action of the locking element is effected as follows. The head of the screw has a distal portion defined by a compression surface and the locking element has a permissive surface that is presented to the compression surface of the head of the screw when the locking element is in the unlocked position. When presented with the permissive surface of the locking element, the compression surface of the head of the screw is unhindered such that the head of the screw is movable in the seat of the coupling element. The locking element also has a confrontational surface that is presented to the compression surface of the head of the screw when the locking element is in the locked position. The confrontational surface of the locking element is defined by a recess that has a recessed surface corresponding to the compression surface of the head of the screw. Therefore, when the locking element is in the locked position: (1) the locking element is compressed within the coupling element, (2) the compression surface of the head of the screw is compressed by the confrontational surface of the locking element such that the curvate proximal portion of the head of the screw is compressed toward the bore of the coupling element and the head of the screw is immovable in the seat of the coupling element, and (3) the compression surface of the head of the screw seats in the recess of the confrontational surface and thereby is biased against retreat from the recess. The compression surface of the head of the screw can be a curvate surface and the confrontational surface of the locking element can extend from an edge of the permissive surface of the locking element and terminate in the recess of the confrontational surface. The confrontational surface of the locking element preferably becomes increasing confrontational and therefore is gradually presented to the compression surface of the head of the screw as the locking element is moved from the unlocked position to the locked position.
Preferably, the rod-receiving channel, presented by the locking element when the locking element is in the unlocked position, is defined by substantially parallel walls. When the locking element is in the locked position, the walls are compressed toward one another such that the rod is compressed within the channel and thereby fixed relative to the coupling element. Preferably, the walls are increasingly compressed toward one another as the locking element is moved from the unlocked position to the locked position.
Preferably, the coupling element has a bore that permits use of the screwdriving tool when the head is seated in the seat, allowing the screwdriving tool to engage the head as described above. Also preferably, the locking element has a bore that permits use of the screwdriving tool when the locking element is disposed within the coupling element in an unlocked position and the head is seated in the seat, allowing the screwdriving tool to engage the head as described above.
A use of the invention can be summarized as follows. First, the locking element is disposed within the coupling element. Then, the locking element is rotated within the coupling element until the locking element is in the unlocked position. The top bores of the locking element and the coupling element will then permit the insertion of the screw into the seat of the coupling element. The surgeon can freely rotate and angulate the screw in the socket to direct the shaft in the clinically appropriate direction for secure lodging in a vertebral bone.
Once the surgeon directs the shaft of the screw in the clinically appropriate direction, he passes the operative end of the screwdriving tool through the top bores to engage the head of the screw to rotate the screw and drive the shaft of the screw into the vertebral bone. A pre-drill hole is usually provided in the bone, into which it is desired that the screw be disposed.
Typically, the surgeon will repeat the process described above using additional screws, coupling elements, and locking elements, lodging the additional screws into the same vertebral bone or other vertebral bones, depending on the clinically desirable result. To that end, the invention encompasses an orthopedic rod implantation apparatus having a plurality of screws and coupling elements of the types described above, and at least one elongate rod. Many implantations will require the use of two rods, however, only one rod or more than two rods may be necessary. The surgeon will, for example, lodge two screws into a first verterbal bone (the screws are positioned laterally adjacent one another in the same bone, one in each pedicle), and lodge two other screws into a second vertebral bone (the screws are positioned laterally adjacent one another in the same bone, one in each pedicle) that may be adjacent the first vertebral bone or may be separated from the first vertebral bone by other vertebral bones that are damaged or unstable.
In a healthy spine, each screw in one of the pairs would roughly align with the corresponding screw in the other pair, because the vertebral bones would be vertically aligned. However, in some cases, the screws will not be aligned because the spine is laterally crooked and the surgeon is installing the implant to realign the vertebral bones. Therefore, in such cases, once the screws are in place, and the associated coupling elements and locking elements are in the unlocked position, presenting respective rod-receiving channels, and free to rotate and angulate with respect to the head of the associated screw, the surgeon inserts a rod into one rod-receiving channel, and thereafter into another rod-receiving channel, so that the rod lines up the verterbral bones affected thereby in a clinically desirable manner, urging them, typically, into vertical alignment. The surgeon similarly inserts a second rod into the remaining two rod receiving channels to balance the alignment forces.
Once the rods are in the rod receiving channels, the surgeon proceeds to move each locking element into the locked position by rotating the locking element. As the locking element is moved to the locked position, the permissive surface, of the locking element, that is presented to the compression surface of the head of the screw when the locking element is in the unlocked position, is removed and the compression surface is increasingly presented with the confrontational surface. This causes the curvate proximal portion of the head of the screw to be increasingly compressed toward the bore until the head is immovable in the seat of the coupling element. Ultimately, the compression surface seats in the recess and thereby is biased against retreat from the recess. This prevents accidental reverse rotational slippage of the locking element from occurring and moving the locking element to the unlocked position.
In addition, as the locking element is moved to the locked position, the rod-receiving channel presented by the locking element when the locking element is in the unlocked position gradually closes. Ultimately, the channel is completely closed. This prevents the rod from laterally exiting the locking element. Further, the walls of the channel are gradually compressed toward one another, until they ultimately secure the rod between them so that the rod is fixed relative to the coupling element. This prevents the rod from axially exiting the locking element.
In addition, as the locking element is moved to the locked position, the locking element itself is gradually compressed within the coupling element until it ultimately is secured within the coupling element. More specifically, as the rod seeks to radially force the walls and apart, the outer surface of the locking element seeks to radially push against the inner surface of the coupling element. However, the structural integrity of the coupling element resists the radial force of the locking element, and the locking element is secured in the coupling element thereby. This prevents the locking element from axially exiting the coupling element.
In this manner, once the locking element is moved to the locked position, the screw, the coupling element, the locking element, and the rod are all fixed relative to one another and to the bone. When all of the locking elements are so positioned, the implant is installed. After the surgeon moves all of the locking elements in the orthopedic rod implantation apparatus to the locked position, he closes the wounds of the patient and the surgery is complete.
Multiple screw and coupling element assemblies are generally necessary to complete the full array of anchoring sites for a rod immobilization system, however, the screw and coupling element assembly of the invention is designed to be compatible with alternative rod systems so that, where necessary, the invention may be employed to rectify the failures of other systems, the implantation of which may have already begun.